Liquid adhesive, such as temperature and/or pressure sensitive adhesive, is applied onto one or more stretched strands of elastic material or a nonwoven substrate during the manufacture of disposable hygiene products such as diapers, adult incontinence products and feminine hygiene products to form various elastic structure which are part of the hygiene product. For example, in a diaper one or more stretched elastic strands are bonded between the backsheet and top sheet around the leg opening so that the diaper snugly fits around the baby's leg. This is commonly referred to as the leg elastic application. One or more stretched elastic strands are also bonded to different areas of the diaper during the construction of the barrier leg cuff and waist band. Two measurements that are commonly used when evaluating the effectiveness of the bond between the stretched elastic strands and the nonwoven substrates are creep resistance and force retraction. Creep resistance is a measure of how well the ends of the elastic strands remain adhered in position with respect to nonwoven substrates. A high level of creep resistance is desirable because creep will cause a strand to decouple from the nonwoven substrate and contract, thereby removing the elasticity and sealing capabilities of the hygiene product. Force retraction is a measure of how much the adhered elastic strand can retract when the tension on the strand is released. A high level of force retraction is also desirable because a low level of force retraction renders the elasticity of the elastic strand and the hygiene product inadequate for its desired purposes, including product comfort and sealing capability. The adhesive is applied to the one or more stretched elastic strands using a non-contact dispensing system or a contact dispensing system.
In the conventional non-contact dispensing system, the adhesive is dispensed as a continuous filament and moved in a controlled pattern by impacting the filament with air. Different types of nozzles are used in conventional non-contact dispensing systems which result in different controlled patterns for the adhesive filament. In one non-contact dispensing system using a spiral nozzle the adhesive filament is moved back and forth in a helical or spiral pattern while it is in the air prior to contacting the stretched elastic strand. The helical or spiral pattern of the adhesive filament has a component in the direction of motion of the stretched elastic strand and another component transverse to the direction of motion of the stretched elastic strand. CF® nozzles (also identified as Controlled Fiberization™ nozzles) and Sure Wrap® nozzles, available from Nordson Corporation of Westlake, Ohio, are spiral nozzles used to form such a helical pattern with an adhesive filament.
In another non-contact dispensing system using a meltblowing nozzle, the adhesive filament is moved back and forth in an oscillating pattern such as a sinusoidal or similar pattern while it is in the air prior to contacting the stretched elastic strand. The oscillating pattern of the adhesive filament is in a plane perpendicular to the motion of the stretched elastic strand.
In non-contact dispensing systems using meltblowing nozzles or spiral nozzles, the adhesive filament must be carefully controlled to ensure that the adhesive filament is dispensed onto the narrow elastic strand and to ensure that the adhesive filament sufficiently wraps around the elastic strand. In this regard, the plurality of air jets used to spiral the adhesive filament in Controlled Fiberization™ and Sure Wrap® nozzles are positioned and angled with a high degree of precision to cause movement of the adhesive filament. If one of the air orifices delivering the air jets is blocked by adhesive material or debris during operation, the overall air pattern is disrupted or unbalanced, which leads to an uncontrolled adhesive filament pattern. The uncontrolled adhesive filament pattern causes an undesirable adhesive deposit onto the strand or away from the strand entirely. The adhesive filament in these non-contact dispensing systems must also exhibit a relatively high viscosity to be adequately controllable in flight. The Sure Wrap® nozzle operates using hot melt adhesives with viscosity in the range of 10,000 centipoises to 15,000 centipoises, and the Controlled Fiberization™ nozzle operates using hot melt adhesives with viscosity in the range of 4000 centipoises to 15,000 centipoises.
Yet another type of non-contact dispensing system uses an adhesive nozzle to extrude a bead of adhesive onto a stretched elastic strand that rotates as it passes by the adhesive nozzle without the use of any process air on the bead of adhesive. The stretched elastic strand is rotated about its axis and moved by a nip roller assembly upstream of the adhesive nozzle. As a result, the continuous filament of adhesive is deposited in a generally spiral pattern along the length of the stretched elastic strand. However, this type of non-contact dispensing system may be impractical because it is difficult to predictably rotate or twist the elastic strand at high production line speeds. Despite the above difficulties, non-contact dispensing systems are widely used because the resulting application of adhesive to the stretched elastic strands results in a high level of both creep resistance and force retraction.
One type of contact dispensing system uses a slit coating nozzle including one or more grooves configured to be filled with extruded adhesive. A stretched elastic strand moving through the grooves will be surrounded with the extruded adhesive in the corresponding groove. Consequently, the stretched elastic strand is coated as the strand moves through the grooves in the slit coating nozzle. Slit coating nozzles do not have the filament control difficulties discussed above because the adhesive is not discharged in an airborne filament. Contact dispensing systems using these slit coating nozzles tend to have difficulties adequately coating the bottom surface of the stretched elastic strand. If the bottom surface of the strand is not adequately coated, there is poor bonding between the elastic strand and a nonwoven substrate, which results in a low level of creep resistance. In order to effectively coat the bottom surface of the elastic strand, the flow rate of adhesive into the groove is commonly increased to a substantial extent, which results in a relatively thick coating of adhesive. This thick coating of adhesive effectively bonds the elastic strand to the substrate and improves the creep resistance, but because the strand is so heavily coated, its ability to retract is impeded and results in poor force retraction. The amount of adhesive dispensed to form the thick coating also tends to undesirably drip off the elastic strand onto other equipment, especially when the production line is stopped. However, a contact dispensing system using a slit coating nozzle to apply adhesive to stretched elastic strands is highly repeatable and consistent.
There is a need, therefore, for a contact adhesive dispensing system, nozzle, and method that supplies optimal coating characteristics of adhesive on an elastic strand, including a high level of creep resistance and a high level of force retraction.